diff --git a/internal/core/unfurl/providers.go b/internal/core/unfurl/providers.go
index d968a9d..46b5d2c 100644
--- a/internal/core/unfurl/providers.go
+++ b/internal/core/unfurl/providers.go
@@ -106,13 +106,24 @@ func fetchOEmbed(ctx context.Context, urlStr string, timeout time.Duration, user
 	return &oembed, nil
 }
 
+// normalizeURL converts protocol-relative URLs to HTTPS
+// Examples:
+//   "//example.com/image.jpg" -> "https://example.com/image.jpg"
+//   "https://example.com/image.jpg" -> "https://example.com/image.jpg" (unchanged)
+func normalizeURL(urlStr string) string {
+	if strings.HasPrefix(urlStr, "//") {
+		return "https:" + urlStr
+	}
+	return urlStr
+}
+
 // mapOEmbedToResult converts oEmbed response to UnfurlResult
 func mapOEmbedToResult(oembed *oEmbedResponse, originalURL string) *UnfurlResult {
 	result := &UnfurlResult{
 		URI:          originalURL,
 		Title:        oembed.Title,
 		Description:  oembed.Description,
-		ThumbnailURL: oembed.ThumbnailURL,
+		ThumbnailURL: normalizeURL(oembed.ThumbnailURL),
 		Provider:     strings.ToLower(oembed.ProviderName),
 		Domain:       extractDomain(originalURL),
 		Width:        oembed.Width,
@@ -186,7 +197,7 @@ func fetchOpenGraph(ctx context.Context, urlStr string, timeout time.Duration, u
 		URI:          urlStr,
 		Title:        og.Title,
 		Description:  og.Description,
-		ThumbnailURL: og.Image,
+		ThumbnailURL: normalizeURL(og.Image),
 		Provider:     "opengraph",
 		Domain:       extractDomain(urlStr),
 	}
diff --git a/internal/core/unfurl/providers_test.go b/internal/core/unfurl/providers_test.go
new file mode 100644
index 0000000..e4174b2
--- /dev/null
+++ b/internal/core/unfurl/providers_test.go
@@ -0,0 +1,54 @@
+package unfurl
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNormalizeURL(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "protocol-relative URL",
+			input:    "//cdn.example.com/image.jpg",
+			expected: "https://cdn.example.com/image.jpg",
+		},
+		{
+			name:     "https URL unchanged",
+			input:    "https://example.com/image.jpg",
+			expected: "https://example.com/image.jpg",
+		},
+		{
+			name:     "http URL unchanged",
+			input:    "http://example.com/image.jpg",
+			expected: "http://example.com/image.jpg",
+		},
+		{
+			name:     "empty string",
+			input:    "",
+			expected: "",
+		},
+		{
+			name:     "protocol-relative with query params",
+			input:    "//cdn.example.com/image.jpg?width=500&height=300",
+			expected: "https://cdn.example.com/image.jpg?width=500&height=300",
+		},
+		{
+			name:     "real Streamable URL",
+			input:    "//cdn-cf-east.streamable.com/image/7kpdft.jpg?Expires=1762932720",
+			expected: "https://cdn-cf-east.streamable.com/image/7kpdft.jpg?Expires=1762932720",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := normalizeURL(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+